1. Field of the Invention
The present invention relates to a matrix driving type transverse electric field type liquid crystal display device of a novel display mode. More particularly, the present invention relates to a matrix driving type transverse electric field type liquid crystal display device of a novel display mode using a homeotropically-oriented nematic liquid crystal material which, as a whole liquid crystal layer, generates spontaneous polarization in the presence of an applied transverse electric field; and to a liquid crystal material which may provide such a liquid crystal display device.
2. Description of the Related Art
A transverse electric field type liquid crystal display device has been conventionally known. The transverse electric field type liquid crystal display device has a liquid crystal layer between a pair of substrates, the liquid crystal layer containing a nematic liquid crystal material oriented parallel to the substrate surface, whereby the device is driven by an applied transverse electric field and by utilizing dielectric anisotropy of the nematic liquid crystal material (e.g., Japanese Laid-Open Patent Publication No. 6-160878).
The transverse electric field type liquid crystal display device has problems in that the aperture ratio is low; a high contrast display is not easily achieved; and the response speed is low. Referring to FIGS. 12A to 12D, the configuration of a conventional transverse electric field type liquid crystal display device and the problems associated therewith will be described.
The liquid crystal display device includes a liquid crystal panel. The liquid crystal panel includes a pair of substrates 203 and 203, transverse electric field electrodes 201 and 202 both of which are provided on one of the substrates, alignment films 204 and 204 each provided on one of the substrates on the liquid crystal layer side thereof, and a liquid crystal layer 210 as a display medium. The liquid crystal display device further includes polarizers 206 and 206 provided external to the liquid crystal panel. In the conventional transverse electric field type liquid crystal display device, in the absence of an applied voltage, liquid crystal molecules 205 contained in the liquid crystal layer 210 are not twisted between the pair of substrates 203 and 203 but are oriented generally parallel to the substrate 203, as shown in FIGS. 12A and 12C. Each of the substrates is provided with the polarizer 206 in such a manner that the direction of a polarization axis 209 of one polarizer 206 is identical to the direction of a molecular axis 208 of the liquid crystal molecules 205 while the direction of a polarization axis 209 of the other polarizer 206 is orthogonal to the direction of the molecular axis 208 of the liquid crystal molecules 205. For example, in the liquid crystal display device of the above-identified publication, the optical axis of linearly-polarized light which has passed through a polarizer provided on a lower substrate (herein-after, referred to as the "lower polarizer"), i.e., the transmission axis of the lower polarizer, is identical to the molecular axis of the liquid crystal molecules. Therefore, there is no birefringence generated by the liquid crystal layer. As a result, the linearly-polarized light coming from the lower side of the liquid crystal panel reaches another polarizer provided on an upper substrate (hereinafter, referred to as the "upper polarizer") without becoming elliptically-polarized light or changing the direction of its optical axis, whereby the linearly-polarized light is blocked by the upper polarizer.
On the other hand, as shown in FIGS. 12B and 12D, when an electric field E is applied in a direction 207 which is at a certain angle with respect to the molecular axis direction 208 of the liquid crystal molecules 205 and is generally parallel to the substrate surface, due to the dielectric anisotropy of the liquid crystal molecules 205, the liquid crystal molecules 205 rotate in a plane parallel to the substrate surface so that the minor axis thereof is orthogonal to the line of electric force. As a result, the optical axis of the linearly-polarized light which has passed through the lower polarizer is shifted with respect to that of the liquid crystal molecules, whereby the light coming from the lower side of the liquid crystal panel passes through the upper polarizer.
The aperture ratio of the conventional transverse electric field type liquid crystal display device is low because the liquid crystal molecules are driven based upon the dielectric anisotropy. In order to maximize the transmission in the conventional liquid crystal display device, the liquid crystal molecules therein must be rotated by 45.degree.. The field strength required for rotating the liquid crystal molecules may vary depending upon the dielectric anisotropy and the elastic constant of the liquid crystal molecules, and the like, but is about 1 V/.mu.m for a commonly-employed liquid crystal material. When a liquid crystal display device having an ordinary pixel size is produced using an ordinary liquid crystal material, the short side of a pixel is about 80 .mu.m long. Accordingly, a driving voltage of about 80 V is required to be applied between the transverse electric field electrodes 201 and 202. However, such a driving voltage, as high as about 80 V, is not practical for an ordinary matrix driving type liquid crystal display device. Therefore, in the conventional liquid crystal display device, an additional electrode (not shown) needs to be provided between the electrodes 201 and 202 in FIGS. 12A to 12D in order to reduce the interval between two electrodes and thus the driving voltage required therebetween. As a result, the additional electrode creates an additional light-blocking portion, thereby lowering the aperture ratio of the liquid crystal display device.
A high contrast display is not easily achieved in the conventional transverse electric field type liquid crystal display device due to the configuration thereof. As described above, in order to block light in the absence of an applied voltage, the direction of the polarization axis of one polarizer (e.g., the transmission axis of the lower polarizer) needs to be identical to the molecular axis direction of the liquid crystal molecules while the direction of the polarization axis of the other polarizer (e.g., the transmission axis of the upper polarizer) needs to be orthogonal to the molecular axis direction of the liquid crystal molecules. For example, if the polarization axis of the lower polarizer is not identical to the molecular axis of the liquid crystal molecules, linearly-polarized light which has passed through the lower polarizer becomes elliptically-polarized light due to the birefringence of the liquid crystal layer, and thus passes through the upper polarizer. Therefore, in order to achieve a high contrast display, it is necessary for the direction of the alignment treatment (e.g., rubbing treatment) for the upper substrate to be precisely identical to that for the lower substrate, for the direction of the alignment treatment to be precisely identical to the polarization axis direction of one of the polarizers, and for the direction of the alignment treatment to be precisely orthogonal to the polarization axis direction of the other polarizer. However, when actually producing a liquid crystal display device, it is very difficult to precisely arrange these components as described above. Accordingly, it is very difficult to achieve a high contrast display with the conventional transverse electric field type liquid crystal display device. Moreover, the productivity in the manufacture of such a liquid crystal display device is very low due to the precise arrangement of the components being required.
The response speed is low in the conventional liquid crystal display device for the following reason. The response speed can be generally classified into two factors, i.e., one factor associated with an increase in the applied voltage and one factor associated with a decrease in the applied voltage. The factor which is of particular importance in the conventional liquid crystal display device is the one associated with a decrease in the applied voltage. In the conventional liquid crystal display device, the liquid crystal molecules are driven based upon the dielectric anisotropy of the liquid crystal molecules. In particular, when a voltage is applied and the field strength thus increases, the liquid crystal molecules are driven with a driving force provided by the electric field acting upon the liquid crystal molecules which have dielectric anisotropy. However, when application of voltage is stopped and the field strength thus decreases, the movement of the liquid crystal molecules is caused only by the restoring force of the elastic liquid crystal molecules (since no driving force is provided by the electric field). As a result, the response speed when the field strength decreases is considerably lower than that when the field strength increases, whereby the total response speed of the liquid crystal display device is relatively low.
On the other hand, another type of transverse electric field type liquid crystal display device has been known having a liquid crystal layer which includes a homeotropically-oriented n-type nematic liquid crystal material (U.S. Pat. No. 3,687,515). However, this patent neither discloses nor suggests how to drive the n-type nematic liquid crystal material. Thus, the technology described therein does not solve any of the above-described problems. Moreover, this technology is limited to a segment type display, and is not capable of performing a high definition display of an arbitrary still image or motion picture. The reasons therefor will be described below. In this technology, in order to provide within a single panel a plurality of regions (pixels) whose transmissions can be individually varied, it is necessary to provide a plurality of electrodes which can be driven individually and to provide the same number of lead wires for introducing a voltage to the electrodes from a source outside of the panel. In such a configuration, in order to improve the definition of a displayed image, a great number of electrodes which can be driven individually must be provided. The number of the lead wires is accordingly increased by the number of additionally-provided electrodes. As a result, the arrangement of the lead wires within the panel, joining terminals connecting to a driving circuit external to the panel, and the like, becomes very complicated, whereby it is impossible to produce a practical liquid crystal display device.
As described above, there is a need for a transverse electric field type liquid crystal display device with the pixel size and driving voltage being in a practical range, and having a high aperture ratio, a high contrast (i.e., a high transmission), a high response speed, and capability of displaying a high definition image.